In the 1800s rudimentary attempts at lowering air temperature were made in hospitals, with fans blowing across tubs of ice. As refrigeration technology advanced, large central plants were established in some US cities to pipe cool air into meat warehouses, beer cellars, bank vaults and archives.

'Manufactured air', as it was known, remained an industrial process until 1902 when Willis Carrier pioneered an independent system, first installed in a Brooklyn printing house to combat humidity. Soon hospitals, schools and factories followed, but the system was still huge, expensive and used toxic ammonia as a coolant. In 1922 Carrier replaced this with dielene and introduced a compressor to make systems more compact. Cinemas, theatres and stores were next to install the luxury of air-con, raising public awareness and demand.

By the 1950s/60s systems had become small enough and affordable for suburban homes across the sweltering central and southern states, and the booming American market set the world standard. Nowadays air-conditioning is no longer a luxury but a common and energy-efficient way to cool AND heat a home!